This invention relates to a process of regenerating a loaded washing liquid which comes from a plant for desulfurizing a gas mixture containing hydrogen and carbon oxides and as load contains H2S, CO, H2, CO2, and nickel carbonyl and/or iron carbonyl, where the washing liquid is reused for desulfurizing after a hot regeneration.
Desulfurizing processes and the associated regeneration of loaded washing liquids are known and described for instance in the German patent 39 22 785 and in the corresponding U.S. Pat. No. 5,085,675. The regeneration of the loaded washing liquids is effected in a known manner by means of one or several of the measures expanding, stripping and heating, where it is also possible to perform two or three of those measures at the same time in one container.
In particular when using physically acting washing liquids, metal carbonyls may lead to problems during the regeneration, as they are converted to sulfides during the heating of the washing agent and form undesired deposits. These carbonyls chiefly include nickel carbonyls, e.g. Ni(CO)4, and iron carbonyls, in particular Fe(CO)5. It is therefore the object underlying the invention to convert the metal carbonyls to sulfides at an appropriate point and remove them from the washing liquid to such an extent that difficulties in the desulfurizing plant and in subsequent plants are avoided. In accordance with the invention this is achieved in the above-mentioned process in that carbon monoxide is at least partly removed from the loaded washing liquid, the washing liquid poor in CO is passed through a reaction and settling zone, a sludge containing nickel sulfide and/or iron sulfide is withdrawn from the reaction and settling zone, and the washing liquid is supplied to the hot regeneration.
In the nickel carbonyls and iron carbonyls the CO is coordinatively bound to the metal atom, so that the carbonyls are converted to sulfides (NiS or FeS) in the loaded washing liquid, as soon as the CO is at least partly removed from the washing liquid. This removal of CO can be effected in various ways, and in particular by means of expanding and/or stripping the washing liquid. The temperature of the loaded washing liquid should favorably be increased, for example, by means of an indirect or direct heat exchange with hot-regenerated washing liquid prior to the removal of CO.
Usually, the desulfurization is effected at a pressure in the range from 10 to 100 bar, so that before the reaction and settling zone the loaded washing liquid can easily be expanded by a pressure difference of at least 3 bar, where a CO-containing expansion gas is released. After this release of CO, the nickel carbonyls and iron carbonyls in the washing solution are gradually converted to insoluble sulfides. It may be expedient to wash the released expansion gas with regenerated washing liquid, so as to keep carbonyls in solution.
Preferably, the pressure in the reaction and settling zone lies in the range from 1 to 20 bar and mostly around at least 3 bar, the temperature usually lies in the range from 0 to 150xc2x0 C. and preferably around at least 40xc2x0 C. Advantageously, it should be ensured that the H2S contained in the loaded washing liquid and also the CO2 are not, or possibly only to a minor extent, released in the reaction and settling zone and only the nickel sulfides and/or iron sulfides are precipitated there. It is therefore expedient to keep the pressure in the reaction and settling zone higher than in the hot regeneration.
To achieve an optimum separation of the metal sulfides from the washing liquid in the reaction and settling zone it is recommended to pass the washing liquid through the reaction and settling zone with a sufficient dwell time, and move the washing liquid as slowly as possible, so that the sulfides can gradually settle. For nickel carbonyls, dwell times in the range from 5 to 80 minutes and mostly 15 to 60 minutes are usually sufficient, whereas iron carbonyls require dwell times of about 1 to 10 hours, preferably at least 3 hours.
The process in accordance with the invention is suited for the treatment of different washing liquids, e.g. methanol, N-methylpyrrolidone (NMP) or dimethyl ether of polyethylene glycol (DMPEG). Usually, these are physically acting washing liquids. The gas mixture to be desulfurized comes from the gasification of solid or liquid carbonaceous materials, e.g. heavy oil, coal, petroleum coke, hydrocarbons containing asphalt, distillation residues or mixtures of these materials. The gasification is effected in a known manner by means of a partial oxidation with air, with oxygen-enriched air or with technically pure oxygen, and mostly by adding steam.